The present invention relates to a protection system of a turbomachine; and more particularly to a method and system for electronic overspeed protection on a turbomachine.
An overspeed condition occurs after the speed of a shaft on a turbomachine exceeds a specified range. During the overspeed condition, a turbomachine typically experiences severe mechanical and thermal stresses that can cause a catastrophic failure. Generally, the turbomachine is equipped protection systems, which attempt to minimize the effects of an overspeed condition. The governor system of the turbomachine generally serves as the primary line of protection system. Upon detecting an overspeed condition, the governor attempts to decrease the speed of the turbomachine. There may also be, generally, two independent secondary lines of protection: a) an overspeed protection system trip, typically set around 110% of operating shaft speed; and b) a emergency protection system trip, typically set around of operating shaft 113% speed. Typically, an overspeed protection system incorporates mechanical, electrical, and software components to safeguard the turbomachine.
Turbomachine operators periodically test the overspeed protection system to determine whether or not the system is functioning properly. There are a few common methods of testing an overspeed protection system.
One method utilizes a multi-channel frequency generator to simulate the speed of the turbomachine. Here, a simulated speed signal, on a single channel, is injected into the control system of the turbomachine. This simulated signal replaces the actual speed signal of the turbomachine. The simulated signal is raised to a speed where the overspeed protection system would trip the turbine. The aforementioned process is repeated with the remaining frequency generator channels.
Another method involves a turbomachine operator manually adjusting the actual turbine speed while the unit is operating. Here, prior to testing, the turbomachine is customarily operating in a full-speed-no-load (FSNL) condition. FSNL is a condition where the turbomachine is at a normal operating speed and not exporting energy to a load such as a generator, compressor, or the like. This method typically involves manually raising the speed until the unit trips. For example, during testing some turbomachine operators raise the speed to 110% of the normal operating speed; thereafter the overspeed system should trip the turbine on overspeed. Typically, the 110% overspeed trip set point may be changed at the direction of a turbine machine operator.
There are a few problems with the current methods of overspeed testing. Regarding the method that utilizes a multi-channel frequency generator, the overspeed protection system is not fully tested. As mentioned, that method only simulates the shaft speed. Hence, the actual shaft speed is not used during the testing. Thus, the method does not determine whether or not some of the mechanical and electrical components of the overspeed system are functioning correctly.
Regarding the method that involves manually adjusting the shaft speed, this method has a few problems. Manually adjusting the shaft speed introduces random and high thermal transients. Also, the method ends in a high-speed trip. Moreover, a trip at a speed well above the normal operating speed, such as 110%, can introduce large mechanical, electrical, and thermal stresses on the turbomachine components, which decreases the maintenance interval. After the trip, a re-start of a turbomachine is required. A re-start delays the export of energy, such as electricity to a utility grid, and a re-start also consumes fuel and other resources. These effects from the trip and subsequent re-start increase the operating expenses of the turbomachine. To avoid the potential problems associated with a high-speed trip, some turbomachine operators lower the overspeed trip set point. For example, the 110% set point is lowered to 100%, in order to reduce the high-speed trip effects. Here, however, the overspeed protection system is not fully tested since the turbomachine is not tripped at the default overspeed trip set point.
These problems, as discussed above, drive turbomachine operators to avoid overspeed testing.
For the foregoing reasons, there is a need for a method and system for testing an overspeed protection system that does not utilize a device that simulates the shaft speed nor cause a trip. The method should automatically adjust the shaft speed and not require a re-start.